Excerpted from: http://www.localharvest.org/features/heritage-pork.jsp
"There was a time not too long ago, before the boom of industrial agriculture, when farmers and smallholders always kept a modest number of what are now known as heritage pig/pork breeds. These were naturally thrifty, hardy animals raised for their meat, bacon and lard. The food chain worked beautifully for all concerned, the pigs lived off the land , on grass, grains and windfalls, and occasionally whey from the churn.
Unfortunately, heritage pork breeds are not suited for the intensive farming techniques being used nowadays, and some of the older breeds are in danger of being lost forever. Fortunately some are being preserved by a few dedicated farmers concerned about the general indifference of the consumer towards heritage breed conservation.
All heritage pork in America comes from pure and cross-bred livestock:
The Berkshire: a first class black pig with excellent lean meat . Occasionally, when commercial white pork becomes too bland and tasteless, some Berkshire genes are used to improve it.
The Tamworth: a red heritage breed producing the best bacon in the U.S., a direct descendant of the wild boars which roamed the forests of Staffordshire. Was introduced to North America around the 1870's. Very outdoorsy and athletic."